On 29/04/21 23:18, Ben Gardon wrote:
This series enables KVM to save memory when using the TDP MMU by waiting to allocate memslot rmaps until they are needed. To do this, KVM tracks whether or not a shadow root has been allocated. In order to get away with not allocating the rmaps, KVM must also be sure to skip operations which iterate over the rmaps. If the TDP MMU is in use and we have not allocated a shadow root, these operations would essentially be op-ops anyway. Skipping the rmap operations has a secondary benefit of avoiding acquiring the MMU lock in write mode in many cases, substantially reducing MMU lock contention. This series was tested on an Intel Skylake machine. With the TDP MMU off and on, this introduced no new failures on kvm-unit-tests or KVM selftests.
Thanks, I only reported some technicalities in the ordering of loads (which matter since the loads happen with SRCU protection only). Apart from this, this looks fine!
Paolo
Changelog: v2: Incorporated feedback from Paolo and Sean Replaced the memslot_assignment_lock with slots_arch_lock, which has a larger critical section. Ben Gardon (7): KVM: x86/mmu: Track if shadow MMU active KVM: x86/mmu: Skip rmap operations if shadow MMU inactive KVM: x86/mmu: Deduplicate rmap freeing KVM: x86/mmu: Factor out allocating memslot rmap KVM: mmu: Refactor memslot copy KVM: mmu: Add slots_arch_lock for memslot arch fields KVM: x86/mmu: Lazily allocate memslot rmaps arch/x86/include/asm/kvm_host.h | 13 +++ arch/x86/kvm/mmu/mmu.c | 153 +++++++++++++++++++++----------- arch/x86/kvm/mmu/mmu_internal.h | 2 + arch/x86/kvm/mmu/tdp_mmu.c | 6 +- arch/x86/kvm/mmu/tdp_mmu.h | 4 +- arch/x86/kvm/x86.c | 110 +++++++++++++++++++---- include/linux/kvm_host.h | 9 ++ virt/kvm/kvm_main.c | 54 ++++++++--- 8 files changed, 264 insertions(+), 87 deletions(-)
